As optoelectronic devices become more important, particularly in the data processing and data transmission areas, the need to maximize the speed at which these devices may be operated becomes increasingly important. Considerable attention has been given to improving the overall operating speed of optoelectronic devices and data systems as a whole. For example, a lengthy discussion of data rate enhancement for optoisolator transmission systems is given in Optoelectronics Applications Manual; Hewlett-Packard Company; Section 3.6.3. The discussion includes a technique of voltage biasing an LED which terminates a transmission line to improve the overall response time.
As another example, U.S. Pat. No. 3,902,060, which issued to J. A. Neuner et al on Aug. 26, 1975, describes a light responsive circuit having a non-linear negative feedback unit which improves several characteristics of photo-coupled data transmission systems including overall response time.
Less emphasis has been placed on directly improving the rise and fall time response of the optoelectronic light generating devices themselves by circuit means. FIG. 7a of the aforementioned U.S. Pat. No. 3,902,060, however, discloses a drive circuit for an LED which overdrives the leading edge of a current pulse to an LED. This improves the rise time of the LED by rapidly charging the junction depletion region of the LED. This overdriving technique cannot be used, however, to improve the fall time characteristic of devices such as LEDs because of this unidirectional nature.
U.S. Pat. No. 4,166,947, which issued to J. P. Dirksen on Sept. 4, 1979 is illustrative of techniques conventionally used to improve the fall time of such devices. This patent discloses shunting an LED in response to a turn-off signal to provide a low impedance path for the elimination of minority carriers in the device. A need exists, however, to further improve the fall time characteristic of optoelectronic devices.